When highschool student Kurono Kei meets up with his childhood friend Masaru Katou at a subway station, their lives take a drastic turn. They both cooperate to save a homeless man who falls onto the tracks, but get run over by the train in the process. Moments later, they appear in an apartment filled with several other people, and a giant black ball in the center of the room. The black ball lights up and informs them that they are now completely under its control. It tells them to go and kill a certain humanoid creature, then it provides them with an arsenal of futuristic weapons, armor, and equipment; and teleports them outside the apartment to a certain alleyway in Tokyo. The people are soon forced to fight for their lives against the creature, each other, and an unexpected visitor. Those who survive are allowed to go free, but they soon discover that it will not be so easy, as the black ball still has control of their fates, and it likes to play games.